Electrostatic copying apparatus with means for preventing contamination of transfer material

ABSTRACT

An electrostatic copying apparatus wherein an idler roller is movable toward and away from a rotary cylindrical xerographic surface carrying powder images for transfer onto a series of discrete sheets of transfer material. A scanning device is located upstream of the transfer station for powder images to effect a delay of the movement of roller toward the xerographic surface until the leading portion of an oncoming sheet is about to reach or reaches the transfer station and to effect a delay of the movement of roller away from the xerographic surface until the trailing portion of a sheet which is in the process of receiving a powder image is about to reach or moves only slightly beyond the transfer station. This insures that the powder images cannot be transferred onto the dielectric layer of the roller and prevents a contamination of the rear sides of next-following sheets.

United States Patent 1191 Szostak et al.

[ 1 ELECTROSTATIC COPYING APPARATUS WITH MEANS FOR PREVENTING CONTAMINATION OF TRANSFER MATERIAL [75] Inventors: Roland Szostak, Gruenwald; Wilm Krueger, Graefelfing, both of Germany [73] Assignee: Agfa-Gevaert Aktiengesellschaft,

Leverkusen, Germany 221 Filed: Oct. 24, 1972 21 Appl. No.: 300,376

{30] Foreign ApplicationPriority Data Oct. 21, 1971 Germany P 21 52 500.8

52] US. Cl.'.. 3 55/3, 96/].4, ll7/17.5, 1 18/637 [51] Int. Cl G03g 15/00 [58] Field of Search 355/3, 47; 96/1.4; 117/175; 118/637 [56] References Cited UNITED STATES PATENTS 2,807,233 9/1957 Fitch 118/637 llll 1 Aug. 7, 1973 3,109,355 11/1963 Ritzerfeld et al. 118/637 X 3,508,823 4/1970 Gundlach et al. 355/3 3,620,616 11/1911 Davidson ass/3 Primary Examiner-Robert P. Greiner Attorney-Michael S. Striker [57] ABSTRACT An electrostatic copying apparatus wherein an idler roller is movable toward and away from a rotary cylindrical xerographic surface carrying powder images for transfer onto a series of discrete sheets of transfer material. A scanning device is located upstream of the transfer station for powder images to effect a delay of the movement of roller toward the xerographic surface until the leading portion of an oncoming sheet is about to reach or reaches the transfer station and to effect a delay of the movement of roller away from the xerographic surface until the trailing portion of a sheet which is in the process of receiving a powder image is about to reach or moves only slightly beyond the transfer station. This insures that the powder images cannot be transferred onto the dielectric layer of the roller and prevents a contamination of the rear sides of nextfollowing sheets.

12 Claims, 3 Drawing Figures PATENTEDAUB 1am SHEEI 2 0F 2 w a v Rm ma Wk Q IL @mw l TRUE WWW m ll l m 7 a? m N Rm ll! 2. L c@\ :6 w R [vs ELECTROSTATIC COPYING APPARATUS WITH MEANS FOR PREVENTING CONTAMINATION OF TRANSFER MATERIAL BACKGROUND OF THE INVENTION The present invention relates to electrostatic copying apparatus in general, and more particularly to improvements in electrostatic copying apparatus wherein powder images formed on a moving xerographic surface are to be transferred onto a series of discrete sheets consisting of transfer material. In such copying apparatus, an electrostatically charged countersurface presses successive increments of a sheet against the selenium layer of the xerographic surface to insure the transfer of powder images.

A drawback of presently known electrostatic copying apparatus is that they are not provided with reliable means for preventing the transfer of powder images onto the countersurface. Thus, if the countersurface remains closely adjacent to the xerographic surface long after the trailing portion of a sheet has been advanced beyond the transfer station between the two surfaces, a powder image is likely to be transferred onto the countersurface during the interval which elapses between the movement of the trailing portion of a preceding sheet beyond and the arrival of the nextfollowing sheet at the transfer station. Any noticeable contamination of the countersurface-is highly undesirable because the latter is likely to smudge the rear side of the next-following sheet.

In certain presently known electrostatic copying apparatus, the mechanism which transports successive sheets toward the transfer station controls movements of the countersurface toward the transfer station. Such operative connection between the sheet transporting mechanism and the displacing mechanism for the countersurface is satisfactory and normally prevents the transfer of powder images onto. the countersurface if the transporting mechanism is capable of transporting successive sheets with a high degree of accuracy, i.e., in such a way that the width of gaps between successive sheets invariably remains within a narrow range. However, if the transporting mechanism cannot advance the sheets with such high degree of accuracy, the width of gaps between successive sheets is likely to vary tosuch an extent that portions of powder images are transferred onto the countersurface which thereupon contaminates the rear side or sides of the nextfollowing sheet or sheets. Furthermore, the just described operative connection between the sheet transporting mechanism and the displacing mechanism for the countersurface cannot prevent a contamination of the countersurface if the apparatus is to transfer powder images onto a series of sheets having different lengths.

SUMMARY OF THE INVENTION An object of the invention is to provide a novel and improved electrostatic copying apparatus wherein the countersurface which biases sheets of transfer material against the xerographic surface during transfer of powder images cannot be contaminated by toner irrespective of whether the width of gaps between successive sheets varies within a wide range and irrespective of whether or not the transporting mechanism feeds a series of sheets having identical or different lengths.

Another object of the invention is to provide an electrostatic copying apparatus with novel and improved means for preventing contamination of the countersurface by toner irregardless of thenature of mechanism which is employed to transport sheets of transfer material between the countersurface and the xerographic surface.

A further object of the invention is to provide novel and improved means for changing the distance between the countersurface and the xerographic surface.

An additional object of the invention is to provide simple, reliable, compact and relatively inexpensive means for preventing a contamination of the countersurface by the material of powder images in an electrostatic copying apparatus.

The invention is embodied in an electrostatic copying apparatus which comprises a driven preferably cylindrical xerographic surface arranged to transport a series of powder images toward a transfer station, a countersurface which is normally spaced apart from the xerographic surface and preferably constitutes an idler roller, means for transporting discrete sheets of transfer material along a path extending between the xerographic surface and the countersurface so that successive sheets advance toward, through and beyond the transfer station, displacing means which is actuatable to move one of the surfaces (preferably the countersurface) toward the other surface to thereby effect at the transfer stationthe transfer of a powder image from the xerographic surface onto one side of a sheet which advances through the transfer station, and control means for'preventing the transfer of powder images onto the countersurface irrespective of the width ofgaps between successive sheets and irrespective of the length of such sheets. The control means comprises scanning .means'which is adjacent to the path of sheets and .is

preferably located immediately upstream of the transfer station to detect the leading portions of successive sheets, and time-delay means for delaying the actuation of displacing means in response to detection of the leading portion of an oncoming sheet by the scanning means until the leading portion of the oncoming sheet reaches or closely approaches the transfer station.

The just described time-delay means is capable of preventing a contamination of the countersurface with the material of powder images irrespective of the width of gaps between successive sheets. In order to further prevent a contamination of the countersurface due to differences between the lengths of successive sheets, the scanning means is preferably constructed and assembled to produce signals in response to detection of trailing portions of successive sheets which are in the process of receiving powder images at the transfer station, and the time-delay means is preferably constructed and assembled to deactivate the displacing means when the thus detected trailing portion of a sheet is immediately adjacent to or reaches or moves only slightly beyond the transfer station.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved copying apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary vertical sectional view of an electrostatic copying apparatus which embodies the invention;

FIG. 2 is a sectional view as seen in the direction of arrows from the line II-II of FIG. 1; and

FIG. 3 is a sectional view as seen in the direction of arrows from the line III-III of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing in detail, there is illustrated a portion of an electrostatic copying apparatus including a housing or frame 38 (FIGS. 2 and 3) supporting a shaft 1 (FIG. 1) for two carriers here shown as twoarmed levers 3, 4 (FIG. 1) which in turn support a second shaft 5 extending in parallelism with the shaft 1. The shaft 5 supports a cylindrical countersurface in the form of an idler roller 9 which is freely rotatable thereon. The shaft 5 is connected to the longer arms of the carriers or levers 3, 4 and the shorter arms of these levers are connected to each other by a rod 10a which is articulately connected to the reciprocable armature 10 of an electromagnet 11. When the electromagnet 11 is energized, the armature 10 moves the levers 3,4 to the operative positions shown in FIG. 1 whereby the levers stress one or more return springs 12. When the electromagnet 11 is deenergized, the armature 10 allows the spring or springs 12 to contact and to respectively return the levers 3, 4 into abutment with stationary stop pins 15, 16 mounted in the frame 38.

In the positions shown in FIG. 1, the levers 3, 4 urge the roller 9 against the underside of a sheet 19 of transfer material whereby the upper side of the sheet 19 bears against the selenium layer 13 on a drum-shaped rotary xerographic surface 14. The outer side of the selenium layer 13 carries a powder image PI which is to be transferred onto the upper side of the sheet 19. The transfer station where the roller 9 can engage the underside of the adjacent sheet 19 is shown at T.S. The parts I, 3, 4, 5, 10, 10a, 11 together constitute a dis placing means which is actuatable on energization of the electromagnet 11 to move the roller 9 toward the selenium layer 13 on the drum 14 so that the roller 9 engages the sheet 19 at the transfer station T.S.

The transporting mechanism for a series of discrete sheets 19 includes two stationary guides 17, 18 which are disposed at the opposite sides of the transfer station T.S. between the xerographic surface 14 and the roller 9. The remaining parts of the transporting mechanism can be constructed and may operate in a manner as disclosed in the commonly owned co-pending application Ser. No. 181,374 filed Sept. 17, 1971. Thus, successive sheets can be removed from a stack and fed in the direction indicated by arrow A to advance along a substantially horizontal path toward, through and beyond the transfer station T.S.

The guide 17 has an aperture 17a for a scanning device 20 which can close a microswitch 21 in response to detection of the leading portion of an oncoming sheet 19. The microswitch 21 controls a time-delay device 22 in the electric circuit 23, 24 of the electromagnet 11. The microswitch 21 closes automatically to thereby energize the time-delay device 22 in response to detection of the leading portion of a sheet 19 by the scanning device 20, and'the microswitch 21 opens automatically when the trailing portion of a sheet 19 has been advanced beyond the scanning device 20. It will be noted that the device 20 scans the path of sheets 19 close to or immediately upstream of the transfer station T.S.

The dielectric layer 9a of the roller 9 is charged by the wire 25 of a corona discharge device which further comprises a grounded housing 26. The wire 25 is connected with a source 27 of high-voltage current. The charging of dielectric layer 9a is made possible because the levers 3, 4 are grounded by way of the shaft 1 and conductor means 28. The xerographic surface 14 is also grounded by means of an elastic contact 29. This insures the transfer of powder image PI from the selenium layer 13 onto the upper sides of successive sheets 19.

Referring to FIGS. 2 and 3, the xerographic surface 14 is mounted on an output element or shaft 32 which is rotated by a drive means, not shown, in a manner well known from the art. The shaft 32 is journalled in the frame 38 and can drive a coaxial shaft 34 by way of a friction clutch 33. The shaft 34 is journalled in the frame 38 and serves to support and rotate a diskshaped cam 35 having a peripheral cam face including a larger-diameter section35a, a smaller-diameter section 35f, and two intermediate sections 35b, 35c which provide for gradual transition between the sections 35a and 35f. The clutch 33 can rotate the cam 35 in a clockwise direction (see the arrow B in FIG. 3). The cam 35 constitutes an actuating means or trip for a microswitch 37 having a movable contact 36 extending into the path of movement of the cam face section 35b so that the microswitch 37 remains closed while the contact 36 engages the cam face section 35a and is allowed to open (e.g., by a suitable spring) when the cam 35 moves the section 35f along the tip of the contact 36.

The frame 38 further supports an electromagnet 39 having an armature including a rod 39a and an arresting element 41 provided with two stops 41a and 41b. When the electromagnet 39 is deenergized, a helical spring 40 which reacts against the frame 38 urges the armature 39a, 41 to the end position shown in FIG. 2 whereby the stop 41b extends into the path of movement of a projection 35e at one side of the cam 35. The other side of the cam 35 carries a second projection 35d which can be engaged and arrested by the stop 41a when the electromagnet 39 is energized to move its armature 39a, 41 against the opposition of the spring 40. The direction in which the armature 39a, 41 moves in response to energization of the electromagnet 39 is indicated by an arrow C.

The operation:

When the xerographic surface 14 rotates about the axis of the shaft 32, the projection 35a of the cam 35 normally bears against the stop 41b so that the friction clutch 33 is prevented from rotating the shaft 34. When the sensing device 20 detects the leading portion of an oncoming sheet 19, the microswitch 21 energizes the electromagnet 39 (which forms part of the time-delay device 22 shown in FIG. 1) which moves the armature 39a, 41 in the direction indicated by arrow C so that the spring 40 stores energy and the stop 41b is disengaged from the projection 35e. Therefore, the shaft 32 is free to rotate the shaft 34 and the cam 35 by way of the friction clutch 33 whereby the cam 35 rotates in the direction indicated by arrow B and moves the section 35b of its cam face against the movable contact 36 of the microswitch 37. This microswitch also forms part of the time-delay device 22 and is closed with a delay which is necessary for the cam 35 to cover the angle shown in FIG. 3. The closing of microswitch 37 coincides with or immediately precedes the arrival of the leading portion of a fresh sheet 19 at the transfer station T.S. of FIG. 1. The microswitch 37 energizes the electromagnet 11 so that the armature pivots the levers 3, 4 and the roller 9 to the position shown in FIG. 1 whereby the dielectric layer 9a of the roller 9 engages the underside of the freshly arrived sheet 19, i.e., the powder image PI cannot be transferred onto the layer 9a because the layers 9a, 13 are normally separated from each other by a sheet 19 whenever the roller 9 is caused to move to the position shown in FIG. 1.

The energized electromagnet 39 of the time-delay device 22 maintains the stop 41a in the path of movement of projection 35d on the cam 35 so that the cam 35 is arrested by the armature 39a, 41 after it completes an angular movement through about one-half of a revolution. As shown in FIG. 3, the projection 35d engages the stop 41a before the cam face section 35a moves beyond the contact 36 of the microswitch 37 so that the electromagnet 11 remains energized and maintains the roller 9 in the operative position of FIG. 1 as' shaft 34 and the cam 35 by way of the friction clutch 33. because the projection 35d is free to bypass the stop 41a. The movable contact 36 allows the microswitch 37 to open and to deenergize the electromagnet 11 with a delay which is required by the cam 35 to cover the distance a 2 shown in FIG. 3. This delay generally corresponds to the interval which is required to advance the trailing portion of a sheet 19 from the opening 17a to the transfer station T.S. The deenergized electromagnet 11 then allows the spring or springs 12 to return the roller 9 to the inoperative position in which the levers 3, 4 respectively abut against the stationary stops 15, 16. It will be noted that the roller 9 moves its dielectric layer 9a away from the selenium layer 13 on the xerographic surface 14 preferably not later than when the trailing portion of a sheet 19 moves through the transfer station T.S. to thus prevent the transfer of toner onto the outer side of the layer 9a. Such transfer could result in smudging of the underside of the next-following sheet or sheets 19.

The cam 35 is automatically arrested in the starting position of FIG. 3 as soon as the projection 35:: returns into engagement with the stop 41!) of the armature 39a, 41. The apparatus is then ready for the next copying operation which can begin as soon as the leading portion of the next-following sheet 19 reaches the scanning device 20. It will be noted that the duration of energization of the electromagnet 11 is not dependent on the width of gaps between successive sheets 19 in the path defined by the guides l7, 18 but only on the length of such sheets. Therefore, a relatively long sheet 19 can be followed by a relatively short sheet, and the width of gaps between successive sheets may vary within any practical range without running the risk that the exposed surface of the dielectric layer 9a would be contaminated with toner due to bodily contact with the selenium layer 13 on the xerographic surface 14. v

The spring or springs 12 of FIG. 1 can be omitted if the longer arms of the levers 3, 4 (with the shaft 5 and roller 9 thereon) are heavy enough to return into abutment with the stops 15, 16 by gravity as soon as the electromagnet 11 is deenergized. The purpose of the spring or springs 12 (and/or the tendency of the roller 9 to move away from the transfer station T.S. by gravity) is to insure that the layer 9a remains spaced apart from the xerographic surface in the event of a power failure.

The scanning device 20, the microswitch 21 and the time-delay device 22 together constitute a control unit which actuates the displacing means 1, 3, 4, 5, 10, 10a, 11 with a predetermined delay following the engagement of scanning device 20 by the leading portion of an oncoming sheet 19 and which deactivates the displacing means with a predetermined delay following movement of the trailing portion of a sheet 19 beyond the scanning device 20.

It is clear that the illustrated time-delay device 22 (including the cam 35, microswitch 37 and electromagnet 39) constitutes but one of a variety of mechanical and/or electrical time-delay means which can be utilizedin the copying apparatus of the present invention to insure that the roller 9 is held in its operative position (as shown in FIG. 1) only when a sheet 19 advances through the transfer station T.S. However, the time-delay device of FIGS. 2 and'3 has been found to be particularly suited for the purpose of preventing a contamination of the dielectric layer 9a because the delay with which thedisplacing means including the electromagnet 11 is respectively actuated and deactivated in response to detection of the leading and trailing portions of sheets 19 can be selected at will by the simple expedient of replacing the cam 35 with a cam having a differently configurated cam face. Moreover, the clutch 33 insures that the cam 35 is driven in synchronism with the xerographic surface 14.

The movement of roller 9 to the position of FIG. 1 as soon as the leading portion of a sheet 9 reaches the transfer station T.S. is desirable to insure that the front portion of the powder image PI is invariably transferred onto such sheet. By the same token, it is desirable to insure that the roller 9 moves away from the station T.S. only when the station T.S. receives the trailing portion of a sheet and not later than when the trailing portion has advanced only slightly beyond the transfer station. Such mode of operation insures the transfer of entire powder images provided that the image is not longer than a sheet 19.

It was found that a very short-lasting contact between the layers 9a and 13 (e.g., while the contacting layers cover a distance of 2-3 millimeterslwill not result in an appreciable contamination of the roller 9. On the other hand, such short-lasting contact prior to arrival of the leading portion of a sheet 19 at the station T.S. and/or subsequent to movement of the trailing portion of a sheet 19 beyond the transfer station T.S. invariably insures the transfer of an entire powder image PI. Consequently, the configuration of the face on the cam 35 is preferably selected in such a way that a short-lasting contact between the layers 9a and 13 might take place at the transfer station T.S. before the leading portion of a sheet reaches the transfer station and after the trailing portion of a sheet has been moved beyond the transfer station.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended 1. In an electrostatic copying apparatus, the combination of a driven xerographic surface arranged to transport a series of powder images toward a transfer station; a countersurface normally spaced apart from said xerographic surface; means for transporting discrete sheets of transfer material along a path extending between said surfaces toward, through and beyond said transfer station; displacing means actuatable to move one of said surfaces toward the other surface to thereby effect at said station the transfer of a powder image from said xerographic surface onto one side of a sheet advancing through said station; and control means for preventing the transfer of powder images onto said countersurface, comprising scanning means adjacent to said path upstream of said transfer station and arranged to detect the leading portions of successive sheets, and time-delay means for delaying the actuation of said displacing means in response to detection of the leading portion of an oncoming sheet by said scanning means until the leading portion of said oncoming sheet at least closely approaches said transfer station.

2. The combination of claim 1, wherein said scanning means is closely adjacent to said transfer station and said one surface is said countersurface.

3. The combination of claim 1, wherein at least said countersurface is an endless cylindrical surface.

4. The combination of claim 1, wherein said one surface is said countersurface, said displacing means comprising carrier means supporting said countersurface and being movable between first and second positions in which said countersurface is respectively remote from and is located at said transfer station, and electromagnet means energizable with a delay determined by said time-delay means to, move said carrier means to said second position.

5. The combination of claim 4, wherein said carrier means comprises lever means pivotable between said first and second positions.

6. The combination of claim 4, further comprising means for biasing said carrier means to said first position.

7. The combination of claim 4, wherein said carrier means tends to assume said first position by gravity.

8. The combination of claim 1, wherein said control means further comprises electric switch means arranged to be closed by said scanning means in response to detection of the leading portion of an oncoming sheet, said time-delay means comprising drive means having a rotary output element, rotary cam means, friction clutch means interposed between said output element and said cam means, and electromagnet means energizable in response to closing of said switch means to thereby permit said clutch means to rotate said cam means through a predetermined distance including a first portion which said cam means covers while maintaining said displacing means in deactivated condition and a second portion which said cam means covers while causing said displacing means to maintain said one surface in contact with a sheet at said transfer station.

9. The combination of claim 8, wherein said xerographic surface is a rotary cylindrical surface receiving torque from said drive means.

10. The combination of claim 8, wherein said cam means comprises a first projection, an angularly offset second projection and a cam face, said electromagnet means having an armature including first stop means located in the path of movement of said first projection in deenergized condition of said electromagnet means and second stop means located in the path of movement of said second projection in the energized condition of said electromagnet means, said displacing means comprising second electromagnet means energizable to move said one surface toward said other surface and said time-delay means further comprising normally open second electric switch means adjacent to said cam face, said cam face being arranged to close said second switch means and to thereby energize said second electromagnet means with a predetermined delay following disengagement of said first stop means from said first projection and before said second projection engages said second stop means.

11. The combination of claim 10, wherein said scanning means is arranged to effect an opening of said first mentioned switch means in response to detection of trailing portions of successive sheets whereby said first mentioned switch means deenergizes said first mentioned electromagnet means so that said second stop means is disengaged from said second projection and said clutch means is free to rotate said cam means to move said first projection into engagement with said first stop means, said cam face being arranged to effect an opening of said second switch means with a predetermined delay following the disengagement of said second stop means from said second projection and prior to reengagement of said first projection with said first stop means.

12. The combination of claim 1, wherein said scanning means is further arranged to detect the trailing portions of successive sheets and said time-delay means is arranged to delay the deactivation of said displacing means in response to detection of the trailing portions of sheets until the trailing portions at least closely approach said transfer station. t t I IF i 

1. In an electrostatic copying apparatus, the combination of a driven xerographic surface arranged to transport a series of powder images toward a transfer station; a countersurface normally spaced apart from said xerographic surface; means for transporting discrete sheets of transfer material along a path extending between said surfaces toward, through and beyond said transfer station; displacing means actuatable to move one of said surfaces toward the other surface to thereby effect at said station the transfer of a powder image from said xerographic surface onto one side of a sheet advancing through said station; and control means for preventing the transfer of powder images onto said countersurface, comprising scanning means adjacent to said path upstream of said transfer station and arranged to detect the leading portions of successive sheets, and time-delay means for delaying the actuAtion of said displacing means in response to detection of the leading portion of an oncoming sheet by said scanning means until the leading portion of said oncoming sheet at least closely approaches said transfer station.
 2. The combination of claim 1, wherein said scanning means is closely adjacent to said transfer station and said one surface is said countersurface.
 3. The combination of claim 1, wherein at least said countersurface is an endless cylindrical surface.
 4. The combination of claim 1, wherein said one surface is said countersurface, said displacing means comprising carrier means supporting said countersurface and being movable between first and second positions in which said countersurface is respectively remote from and is located at said transfer station, and electromagnet means energizable with a delay determined by said time-delay means to move said carrier means to said second position.
 5. The combination of claim 4, wherein said carrier means comprises lever means pivotable between said first and second positions.
 6. The combination of claim 4, further comprising means for biasing said carrier means to said first position.
 7. The combination of claim 4, wherein said carrier means tends to assume said first position by gravity.
 8. The combination of claim 1, wherein said control means further comprises electric switch means arranged to be closed by said scanning means in response to detection of the leading portion of an oncoming sheet, said time-delay means comprising drive means having a rotary output element, rotary cam means, friction clutch means interposed between said output element and said cam means, and electromagnet means energizable in response to closing of said switch means to thereby permit said clutch means to rotate said cam means through a predetermined distance including a first portion which said cam means covers while maintaining said displacing means in deactivated condition and a second portion which said cam means covers while causing said displacing means to maintain said one surface in contact with a sheet at said transfer station.
 9. The combination of claim 8, wherein said xerographic surface is a rotary cylindrical surface receiving torque from said drive means.
 10. The combination of claim 8, wherein said cam means comprises a first projection, an angularly offset second projection and a cam face, said electromagnet means having an armature including first stop means located in the path of movement of said first projection in deenergized condition of said electromagnet means and second stop means located in the path of movement of said second projection in the energized condition of said electromagnet means, said displacing means comprising second electromagnet means energizable to move said one surface toward said other surface and said time-delay means further comprising normally open second electric switch means adjacent to said cam face, said cam face being arranged to close said second switch means and to thereby energize said second electromagnet means with a predetermined delay following disengagement of said first stop means from said first projection and before said second projection engages said second stop means.
 11. The combination of claim 10, wherein said scanning means is arranged to effect an opening of said first mentioned switch means in response to detection of trailing portions of successive sheets whereby said first mentioned switch means deenergizes said first mentioned electromagnet means so that said second stop means is disengaged from said second projection and said clutch means is free to rotate said cam means to move said first projection into engagement with said first stop means, said cam face being arranged to effect an opening of said second switch means with a predetermined delay following the disengagement of said second stop means from said second projection and prior to reengagement of said first projection with said first stop means.
 12. The combination of claim 1, wherein said scanning means is further arranged to detect the trailing portions of successive sheets and said time-delay means is arranged to delay the deactivation of said displacing means in response to detection of the trailing portions of sheets until the trailing portions at least closely approach said transfer station. 